Currently, numerous frequency bands and standards are used in different regions around the world for cellular phones. Many cellular service providers support more than one region of the world. Consequently, service providers require cellular phones that can be used throughout their system. However, since phone manufacturers produce phones for number of service providers, “universal” cellular phones that are compatible to principal frequency bands and standards used throughout the world are being developed.
There are five principal frequency bands used in Europe, much of Asia and North America. These principal frequency bands are 850, 900, Digital Communications System (DCS), Personal Communications Services (PCS), and Universal Mobile Telecommunications System (UMTS). The regions where the principal frequency bands are used and the associated transmit and receive frequencies are set forth in the following table.
Band Name/RegionTransmit FrequencyReceive Frequency850/North America824 to 849 MHz869 to 894 MHz900/Europe and Asia880 to 915 MHz925 to 960 MHzDCS/Europe and Asia1710 to 1785 MHz1805 to 1880 MHzPCS/North America1850 to 1910 MHz1930 to 1990 MHzUMTS/Europe and Asia1920 to 1980 MHz2110 to 2170 MHzThese bands are also typically, but not exclusively, used elsewhere in the world.
However, the bulk of the cell band communication occurs in these bands. Cellular phones currently in production support only some of these bands.
In addition to the different frequency bands, the universal cellular phones must support different standards being used. In some standards, such as Global System for Mobile communications (GSM), communication signals are transmitted and received in an alternating fashion, so called half duplex. In other standards, such as Wideband Code Division Multiple Access (WCDMA), communication signals are transmitted and received in a simultaneous manner, so called full duplex.
Another design consideration for universal cellular phones is the overall size of the phones. Smaller cellular phones are desired in the market place, and thus, the universal cellular phones must be small in size. An antenna is among the largest components in a cellular phone. In order to minimize the size of cellular phones, smaller antennas are being used in cellular phones. However, as an antenna is made smaller, the frequency response of the antenna becomes more difficult to manage. Small antennas are good only for signals higher in frequency or are inherently narrowband.
A small antenna can be made to respond to multiple frequencies, but it is easier if the frequencies are harmonically related. This is reflected in 900 and DCS bands, which can both be accommodated using a single small harmonic antenna. Furthermore, it turns out that 850 and PCS bands are close enough to 900 and DCS bands that the same antenna can be used for all four bands. A switch is used to selectively connect the harmonic antenna to a desired transmit or receive signal path for 850, 900, DCS and PCS bands. Power amplifiers are sufficiently broadband that each can handle two bands. Thus, one amplifier can be used for both 850 and 900 bands, and another amplifier can be used for both DCS and PCS bands. Consequently, only two switch states are required for four transmit states. Receivers, however, have input band limiting filters, and so typically require separate switch states for each band. These can all be combined into one branch, as shown in FIG. 1. As the performance of the switch is a function of the number of throws, such a scheme minimizes the loss in the transmit state, with a minor increase in the receive states. The receive states can be reduced to two by utilizing a combining scheme at the receive filters. Doing so reduces the switch loss in the receive path, but increases the loss of the filters, and so presents little net benefit. Two transmit signal paths and four receive signal paths are required to accommodate 850, 900, DCS and PCS bands. Thus, the switch needs to be a three-by-four switch to be able to connect the antenna to one of the signal paths. However, UMTS band cannot be effectively accommodated using this harmonic antenna. Therefore, a typical prior art universal cellular phone uses two antennas to accommodate all five principal bands, one harmonic antenna for the 850, 900, DCS and PCS bands and one narrow-banded antenna for UMTS band.
The components required by this architecture, including the switch, can be used as well in any cellular phone that is designed for some subset of all these bands. If UMTS band is not required, then the components for UMTS band are not populated in the cellular phone. As an example, if a cellular phone is aimed for North America, the UMTS components can be replaced by PCS WCDMA components.
However, in North America, service providers require cellular phones to support WCDMA in both PCS and 850 bands. The 850 band for WCDMA (“850 WCDMA”) cannot utilize the narrow-banded antenna used for UMTS or PCS WCDMA, but rather must use the harmonic antenna used for 850, 900, DCS and PCS bands. Consequently, the switch must now be modified to a four-by-four switch, as shown in FIG. 2, which makes the switch more expensive and more lossy. Furthermore, if the same modified switch is used for all cellular phones, then these problems will exist even in Europe and Asia.
In view of the above-described concerns, what is needed is a cellular phone and method for transmitting and receiving signals that can accommodate GSM in 850, 900, DCS and PCS bands and WCDMA in 850, PCS and UMTS bands with less expensive and/or better performing components than the compatible components used in prior art cellular phones.